xfs: Improve CONFIG_XFS_RT Kconfig help

Improve the description of the XFS_RT configuration option to document
that this option is required for zoned block devices.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: damien-lemoal

fs/xfs/Kconfig

@@ -119,6 +119,15 @@ config XFS_RT
 
 	  See the xfs man page in section 5 for additional information.
 
+	  This option is mandatory to support zoned block devices. For these
+	  devices, the realtime subvolume must be backed by a zoned block
+	  device and a regular block device used as the main device (for
+	  metadata). If the zoned block device is a host-managed SMR hard-disk
+	  containing conventional zones at the beginning of its address space,
+	  XFS will use the disk conventional zones as the main device and the
+	  remaining sequential write required zones as the backing storage for
+	  the realtime subvolume.
+
 	  If unsure, say N.
 
 config XFS_DRAIN_INTENTS